To date, organic EL (electro-luminescence) devices including a plurality of emitting layers each of which emits light of a different wavelength are known. Such organic EL devices are also known to provide light of mixed color in which the light emitted by the emitting layers are mixed together. An example of such organic EL devices includes a red emitting layer, green emitting layer and blue emitting layer, and provides white-color light in which the light emitted by the emitting layers are mixed together.
Excited state of organic compounds encompasses excited singlet state and excited triplet state. Emission obtained from excited singlet state is called fluorescence while emission obtained from excited triplet state is called phosphorescence. Excited singlet state and excited triplet state typically occur in a ratio of 1:3.
In typical organic EL devices, fluorescent materials for mainly emitting fluorescence have been used. According to organic EL devices that use such fluorescence emission, excited singlet, which occupies 25% of the excitation generated in emitting layers, only contributes to the emission, so that excited triplet occupying the remaining 75% is deactivated without emitting.
In order to enhance luminous efficiency of organic EL devices, developments are being made on phosphorescent materials for emitting phosphorescence, i.e., emission from excited triplet (e.g., patent document 1). Reports have been made on, for instance, red-emitting phosphorescent materials and green-emitting phosphorescent material. It should be noted that no practically-applicable blue-emitting phosphorescent material has been obtained.
With application of the above phosphorescent materials, organic EL devices for mixed-color emission are capable of enhancing luminous efficiency.
For instance, a known organic EL device provides white-color emission with use of blue-emitting fluorescent materials and red-to-green-emitting phosphorescent materials. According to such a device, enhancement of quantum efficiency in emission of red to green color contributes to enhancement of luminous efficiency of the entire organic EL device.
In such an organic EL device for mixed-color emission, however, while emission is obtained from excited triplet in the red to green emitting layers, excited triplet in the blue emitting layer is deactivated without contributing to emission due to small energy gap.
Accordingly, proposals have been made on an organic EL device in which excited triplet in the blue emitting layer is diffused in the red and green emitting layers so that red and green phosphorescence is obtained therefrom (e.g., patent documents 2 and 3 and non-patent document 1). According to such an organic EL device, the energy of excited triplet in the blue emitting layer, which has been typically to be deactivated without contributing to emission, can be utilized for obtaining red and green phosphorescence. Thus, luminous efficiency of the entire organic EL device can be enhanced.
The organic EL device disclosed in the non-patent document 1 includes a blue fluorescent-emitting layer, blocking layer, red phosphorescent-emitting layer, green phosphorescent-emitting layer, blocking layer and blue fluorescent-emitting layer in this order. According to the document, blue fluorescence is obtained from the excited singlet in the blue fluorescent-emitting layers, and the excited triplet in the blue fluorescent-emitting layers is diffused in the red and green phosphorescent-emitting layers via the blocking layers. Then, excited triplet in the red and green phosphorescent-emitting layers is generated, from which red and green phosphorescence is obtained. According to the report, the blue fluorescence and the red and green phosphorescence are mixed together, thereby providing white-color emission as a whole.
In the organic EL device disclosed in the non-patent document 1, however, holes and electrons are injected into not only the blue fluorescent-emitting layers but also red and green phosphorescent-emitting layers, thereby generating exciton. In this organic EL device, generation of sufficient exciton in the blue fluorescent-emitting layer is prevented, so that the blue emission intensity is reduced. Thus, the organic EL device may not be able to produce white-color emission as the whole.
According to a known exemplary organic EL device, a charge blocking layer having a smaller affinity level than a first emitting layer for blue emission is provided between the first emitting layer and a second emitting layer for another color emission. According to the proposal, the charge blocking layer can facilitate the retention of electrons in the first emitting layer, thereby increasing the probability of the exciton generation in the first emitting layer and raising the intensity of the blue emission (e.g., patent document 4).
The above organic EL device, however, concentrates the charges in the first emitting layer (i.e., the layer of which blue-color emission is weak in terms of intensity) for the sake of balancing the blue-color emission with the red-color emission, so that the charge blocking layer is defined only in terms of affinity level.
Thus, even if a phosphorescent material is used in the second emitting layer, the above device configuration is not adoptable in a device configured to obtain the phosphorescence of the second emitting layer from triplet energy transferred from the first emitting layer.
According to the organic EL devices disclosed in the patent documents 2 and 3, fluorescent host contained in the fluorescent emitting layer exhibits a small triplet energy gap, and the triplet energy is not transferred to the green phosphorescent material. Thus, green phosphorescence is not obtainable.
Further, a patent document 5 discloses in FIG. 6 an organic EL device for emitting white light with use of triplet energy transfer. However, α-NPD (4,4′-bis[N-(1-naphthyl)-N-phenyl-amino]-biphenyl) used in the device exhibits a smaller triplet energy gap than a green phosphorescent material (e.g., Ir(ppy)3(fac-tris(2-phenylpyridine) iridium)). Thus, green phosphorescence is not obtainable.    Patent Document 1: US2002/182441    Patent Document 2: WO2006/038020    Patent Document 3: WO2004/060026    Patent Document 4: WO2005/099313    Patent Document 5: US2002/197511    Non-Patent Document 1: nature vol 440 p. 908